The present invention relates to a method of and an apparatus for perforating a printed circuit board. More particularly, the present invention is concerned with a method of and an apparatus for forming connection holes in a multi-layered printed circuit board for providing electrical connections between internal wiring patterns and wiring patterns on the surface of the printed circuit board.
Japanese Patent Unexamined Publication No. 58-64097, for example, discloses a method of forming connection holes in a printed circuit board for the purpose of providing electrical connections between internal wiring patterns and wiring patterns on the surface of the printed circuit board.
In this method, pilot holes are formed by etching in portions of the outer copper foil layer where the connection holes are to be formed. The pilot holes have a diameter slightly smaller than that of a CO.sub.2 gas laser beam which is to be used for forming the connection holes. Namely, the CO.sub.2 gas laser beam is applied through these pilot holes to remove portions of the resin layers around the etched pilot holes thereby forming the connection hole.
This perforating process includes steps of etching, perforation, plating and etching. Thus, etching is conducted twice so that the efficiency of the work is impaired undesirably.
Furthermore, since the etching for forming the pilot holes and the perforation for forming the connection holes are conducted in separate steps, the precision may be impaired seriously due to accumulation of errors incurred during these steps. Namely, the shape of the connection holes tends to fluctuate due to deviation of the perforating laser beam from the position of the pilot hole formed by etching. In consequence, the area of electrical contact is undesirably reduced at the bottom of the connection hole, with the result that the reliability of the electrical connection between the external wiring pattern and the internal wiring pattern is reduced.
For instance, when a connection hole having an opening diameter of 150 .mu.m in the outer layer and a bottom diameter of 100 to 120 .mu.m is to be formed, the bottom diameter of the connection hole may be reduced to 50 to 70 .mu.m if the deviation of the laser beam is 50 .mu.m or greater, with the result that the reliability of the electrical connection is impaired seriously.
FIG. 5 schematically illustrates the process steps in a conventional method for perforating a printed circuit board.
FIG. 5(a) shows the structure of a typical known printed circuit board. The printed circuit board has an outer copper foil layer 1, an inner copper foil layer 2, cloth layers 3, 3' formed of bundles of glass fibers, and resin layers 4, 4', 4". In order to ensure sufficiently large bonding strengths between the outer and inner copper foil layers 1, 2 and the resin layers 3, 3', 4, 4', 4", bonding resin regions of thicknesses a and b essentially exist in the boundaries between these layers. Numeral 5 denotes a pilot hole formed by etching for a subsequent irradiation with a laser beam.
Portions of the glass fiber layers 3, 3' and the resin layers 4, 4', 4' in the region around the pilot hole 5 are removed by the subsequent application of the laser beam. The amounts of removal of the glass fiber layers and the resin layers, however, are not equal because of various factors such as the energy absorption capacity, rate of absorption of energy, difference in the energy density between the in-focus position and the out-of-focus position, difference in the irradiation time, and so forth. For instance, it is assumed here that the laser beam is a Gauss beam and focused on the surface of the inner layer through the resin layer to be removed having a considerably large thickness H. In such a case, although the bottom portion of the connection hole is finished satisfactorily, defects such as carbonization of the resin, projection of glass fibers in an acicular form at a height of W.sub.1 and so forth are encountered in the surface region of the resin around the inlet opening of the connection hole, as shown in FIGS. 5(b) and 5(c). In the worst case, a bulk of glass fibers of a large size undesirably remain on the surface of the hole as denoted by 6. These defects are attributable to the fact that the surface region of the resin is removed taking an intermediate step of melting rather than by sublimation, because such region is subjected for a long time to a beam of a lower energy density than the bottom region.
When the laser beam is applied for a long time, a problem is encountered in the steps shown in FIGS. 5(a) and 5(b) in that the resin is undesirably recessed as indicated at W.sub.2 due to reflection and diffusion of the laser beam at the surface of the inner copper foil layer which has been made acicular for the purpose of plating precipitation. Too long irradiation time also poses a problem in that the resin 4" which is evaporated at the center of the bottom of the hole due to excess beam energy undesirably raises the inner foil layer. In addition, carbonization of the resin 4" also was caused.
The carbonized portion of the resin on the wall of the hole can be removed by a suitable chemical processing. Such a chemical processing, however, undesirably allows the glass fibers to project from the resin in greater amounts, thereby making it difficult to impregnate the hole wall with a plating solution, with the result that the reliability of the connection formed by the plating through the connection hole is seriously impaired. In addition, it is impossible to remove the carbonized portion of the resin 4". In order to obviate these problems, it has been proposed to use a special resin material instead of the glass fibers to enable perforation by a laser beam. Printed circuit board employing such a special resin material, however, could not be used practically because of inferior physical properties as compared with conventional printed circuit boards.
In addition, the conventional method could not be applied to formation of connection hole in the inner copper foil layer of a multi-layered printed circuit board of the type shown in FIG. 2(b), because the laser beam is reflected by the inner copper foil layer.